feat(skills): add frontend accessibility skill (#2048)

Adds the frontend-a11y skill for React and Next.js accessibility patterns. Pre-validated against current main with validate-skills and markdownlint.
2026-05-31 22:28:22 +08:00 · 2026-05-26 03:09:14 +09:00 · 2026-05-26 03:09:14 +09:00 · 8fb728d7eb
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 ---
 name: frontend-a11y
 description: >
  Accessibility patterns for React and Next.js — semantic HTML, ARIA attributes,
  form labeling, keyboard navigation, focus management, and screen reader support.
  Use when building any interactive UI component or form.
 origin: community
 ---
 # Frontend Accessibility Patterns
 Practical accessibility patterns for React and Next.js. Covers the issues most commonly flagged in code review: missing form labels, incorrect ARIA usage, non-semantic interactive elements, and broken keyboard navigation.
 ## When to Activate
 - Building or reviewing form components (`<input>`, `<select>`, `<textarea>`)
 - Creating interactive elements (modals, dropdowns, tooltips, tabs)
 - Using `<div>` or `<span>` with `onClick`
 - Adding `aria-*` attributes to any element
 - Implementing keyboard navigation or focus management
 - Receiving accessibility feedback from code review tools (CodeRabbit, ESLint a11y)
 - Building components that must support screen readers
 ## Form Accessibility
 Missing `htmlFor` / `id` pairing and disconnected error messages are the most common issues flagged in code review.
 ### Label Connection
 ```tsx
 // BAD: label has no connection to input — screen readers cannot associate them
 <label>Email</label>
 <input type="email" />
 // GOOD: htmlFor matches input id
 <label htmlFor="email">Email</label>
 <input id="email" type="email" />
 ```
 ### Required Fields
 ```tsx
 // BAD: visual-only asterisk conveys nothing to screen readers
 <label htmlFor="email">Email *</label>
 <input id="email" type="email" />
 // GOOD: required enables native browser validation; aria-required signals it to screen readers
 <label htmlFor="email">
  Email <span aria-hidden="true">*</span>
 </label>
 <input id="email" type="email" required aria-required="true" />
 ```
 ### Error Messages
 ```tsx
 // BAD: error text exists visually but is not linked to the input
 <input id="email" type="email" />
 <span className="error">Invalid email address</span>
 // GOOD: aria-describedby connects input to its error message
 // aria-invalid signals the invalid state to screen readers
 <input
  id="email"
  type="email"
  aria-describedby="email-error"
  aria-invalid={!!error}
 />
 {error && (
  <span id="email-error" role="alert">
    {error}
  </span>
 )}
 ```
 ### Complete Accessible Form
 ```tsx
 interface LoginFormProps {
  onSubmit: (email: string, password: string) => void;
 }
 export function LoginForm({ onSubmit }: LoginFormProps) {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');
  const [errors, setErrors] = useState<{ email?: string; password?: string }>({});
  const handleSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    const newErrors: typeof errors = {};
    if (!email) newErrors.email = 'Email is required';
    if (!password) newErrors.password = 'Password is required';
    if (Object.keys(newErrors).length) {
      setErrors(newErrors);
      return;
    }
    onSubmit(email, password);
  };
  return (
    <form onSubmit={handleSubmit} noValidate>
      <div>
        <label htmlFor="email">
          Email <span aria-hidden="true">*</span>
        </label>
        <input
          id="email"
          type="email"
          value={email}
          onChange={e => setEmail(e.target.value)}
          aria-required="true"
          aria-describedby={errors.email ? 'email-error' : undefined}
          aria-invalid={!!errors.email}
          autoComplete="email"
        />
        {errors.email && (
          <span id="email-error" role="alert">
            {errors.email}
          </span>
        )}
      </div>
      <div>
        <label htmlFor="password">
          Password <span aria-hidden="true">*</span>
        </label>
        <input
          id="password"
          type="password"
          value={password}
          onChange={e => setPassword(e.target.value)}
          aria-required="true"
          aria-describedby={errors.password ? 'password-error' : undefined}
          aria-invalid={!!errors.password}
          autoComplete="current-password"
        />
        {errors.password && (
          <span id="password-error" role="alert">
            {errors.password}
          </span>
        )}
      </div>
      <button type="submit">Log in</button>
    </form>
  );
 }
 ```
 ## Semantic HTML
 Use the element that matches the intent. Screen readers and keyboard users depend on native semantics.
 ```tsx
 // BAD: div has no role, no keyboard support, no accessible name
 <div onClick={handleClick}>Submit</div>
 // GOOD: button is focusable, activates on Enter/Space, announces as "button"
 <button type="button" onClick={handleClick}>Submit</button>
 ```
 ```tsx
 // BAD: non-semantic navigation
 <div onClick={() => navigate('/home')}>Home</div>
 // GOOD: anchor supports right-click, middle-click, and keyboard navigation
 <a href="/home">Home</a>
 ```
 ```tsx
 // BAD: heading hierarchy skipped (h1 to h4)
 <h1>Dashboard</h1>
 <h4>Recent Activity</h4>
 // GOOD: sequential heading levels
 <h1>Dashboard</h1>
 <h2>Recent Activity</h2>
 ```
 ## ARIA Attributes
 Use ARIA only when native HTML semantics are insufficient. Wrong ARIA is worse than no ARIA.
 ### aria-label vs aria-labelledby
 ```tsx
 // aria-label: inline string label — use when no visible label text exists
 <button aria-label="Close modal">
  <XIcon />
 </button>
 // aria-labelledby: references another element's text — use when a visible label exists
 <section aria-labelledby="section-title">
  <h2 id="section-title">Recent Orders</h2>
  {/* content */}
 </section>
 ```
 ### aria-describedby
 ```tsx
 // Provides supplementary description beyond the label
 <button
  aria-describedby="delete-warning"
  onClick={handleDelete}
 >
  Delete account
 </button>
 <p id="delete-warning">This action cannot be undone.</p>
 ```
 ### aria-live for Dynamic Content
 ```tsx
 // Use aria-live to announce content that updates without a page reload
 // polite: waits for user to finish current action before announcing
 // assertive: interrupts immediately — use only for urgent errors
 export function StatusMessage({ message, isError }: { message: string; isError?: boolean }) {
  return (
    <div role="status" aria-live={isError ? 'assertive' : 'polite'} aria-atomic="true">
      {message}
    </div>
  );
 }
 ```
 ### aria-expanded and aria-controls
 ```tsx
 export function Accordion({ title, children }: { title: string; children: React.ReactNode }) {
  const [isOpen, setIsOpen] = useState(false);
  const contentId = useId();
  return (
    <div>
      <button aria-expanded={isOpen} aria-controls={contentId} onClick={() => setIsOpen(prev => !prev)}>
        {title}
      </button>
      <div id={contentId} hidden={!isOpen}>
        {children}
      </div>
    </div>
  );
 }
 ```
 ## Keyboard Navigation
 Every interactive element must be reachable and operable by keyboard alone.
 ### Custom Dropdown
 ```tsx
 export function Dropdown({ options, onSelect }: { options: string[]; onSelect: (value: string) => void }) {
  const [isOpen, setIsOpen] = useState(false);
  const [activeIndex, setActiveIndex] = useState(0);
  const listId = useId();
  if (!options.length) return null;
  const handleKeyDown = (e: React.KeyboardEvent) => {
    switch (e.key) {
      case 'ArrowDown':
        e.preventDefault();
        setActiveIndex(i => Math.min(i + 1, options.length - 1));
        break;
      case 'ArrowUp':
        e.preventDefault();
        setActiveIndex(i => Math.max(i - 1, 0));
        break;
      case 'Enter':
      case ' ':
        e.preventDefault();
        if (isOpen) onSelect(options[activeIndex]);
        setIsOpen(prev => !prev);
        break;
      case 'Escape':
        setIsOpen(false);
        break;
    }
  };
  return (
    <div
      role="combobox"
      aria-expanded={isOpen}
      aria-haspopup="listbox"
      aria-controls={listId}
      tabIndex={0}
      onKeyDown={handleKeyDown}
      onClick={() => setIsOpen(prev => !prev)}
    >
      <span>{options[activeIndex]}</span>
      {isOpen && (
        <ul id={listId} role="listbox">
          {options.map((option, index) => (
            <li
              key={option}
              role="option"
              aria-selected={index === activeIndex}
              onClick={() => {
                onSelect(option);
                setIsOpen(false);
              }}
            >
              {option}
            </li>
          ))}
        </ul>
      )}
    </div>
  );
 }
 ```
 ## Focus Management
 Focus must move logically when UI state changes — especially for modals and route transitions.
 ### Modal Focus Restoration
 > This example covers initial focus and restoration. For a full focus trap (Tab/Shift+Tab cycling within the modal), use a library like [`focus-trap-react`](https://github.com/focus-trap/focus-trap-react) which handles edge cases like dynamic content and nested portals.
 ```tsx
 export function Modal({ isOpen, onClose, title, children }: { isOpen: boolean; onClose: () => void; title: string; children: React.ReactNode }) {
  const modalRef = useRef<HTMLDivElement>(null);
  const previousFocusRef = useRef<HTMLElement | null>(null);
  useEffect(() => {
    if (isOpen) {
      // Save currently focused element and move focus into modal
      previousFocusRef.current = document.activeElement as HTMLElement;
      modalRef.current?.focus();
    } else {
      // Restore focus to the element that opened the modal
      previousFocusRef.current?.focus();
    }
  }, [isOpen]);
  if (!isOpen) return null;
  return (
    <div ref={modalRef} role="dialog" aria-modal="true" aria-labelledby="modal-title" tabIndex={-1} onKeyDown={e => e.key === 'Escape' && onClose()}>
      <h2 id="modal-title">{title}</h2>
      {children}
      <button onClick={onClose}>Close</button>
    </div>
  );
 }
 ```
 ## Images and Icons
 ```tsx
 // BAD: decorative icon announced as unlabeled image
 <img src="/icon.svg" />
 // GOOD: decorative image hidden from screen readers
 <img src="/decoration.png" alt="" aria-hidden="true" />
 // GOOD: meaningful image with descriptive alt text
 <img src="/chart.png" alt="Monthly revenue increased 23% from January to March" />
 // GOOD: icon button with accessible label
 <button aria-label="Delete item">
  <TrashIcon aria-hidden="true" />
 </button>
 ```
 ## Reduced Motion
 Respect users who have requested reduced motion in their OS settings.
 ```tsx
 export function useReducedMotion(): boolean {
  const [prefersReduced, setPrefersReduced] = useState(false);
  useEffect(() => {
    const mq = window.matchMedia('(prefers-reduced-motion: reduce)');
    setPrefersReduced(mq.matches);
    const handler = (e: MediaQueryListEvent) => setPrefersReduced(e.matches);
    mq.addEventListener('change', handler);
    return () => mq.removeEventListener('change', handler);
  }, []);
  return prefersReduced;
 }
 // Usage
 export function AnimatedCard({ children }: { children: React.ReactNode }) {
  const reduceMotion = useReducedMotion();
  return (
    <div
      style={{
        transition: reduceMotion ? 'none' : 'transform 300ms ease'
      }}
    >
      {children}
    </div>
  );
 }
 ```
 ## Anti-Patterns
 ```tsx
 // BAD: onClick on non-interactive element with no keyboard support
 <div onClick={handleClick}>Click me</div>
 // BAD: aria-label on a div that has no role
 <div aria-label="Navigation">...</div>
 // BAD: placeholder used as a substitute for label
 <input placeholder="Enter your email" />
 // BAD: positive tabIndex creates unpredictable tab order
 <button tabIndex={3}>Submit</button>
 // BAD: aria-hidden on a focusable element — keyboard users get trapped
 <button aria-hidden="true">Open</button>
 // BAD: role="button" on div without keyboard handler
 <div role="button" onClick={handleClick}>Submit</div>
 // Missing: tabIndex={0}, onKeyDown for Enter/Space
 ```
 ## Checklist
 Before submitting any interactive component for review:
 - [ ] Every `<input>`, `<select>`, and `<textarea>` has a connected `<label>` via `htmlFor`/`id`
 - [ ] Error messages are linked with `aria-describedby` and marked `role="alert"`
 - [ ] No `onClick` on `<div>` or `<span>` without `role`, `tabIndex`, and `onKeyDown`
 - [ ] Icon-only buttons have `aria-label`
 - [ ] Decorative images use `alt=""` and `aria-hidden="true"`
 - [ ] Modals restore focus on close (for full focus trapping with Tab/Shift+Tab cycling, use a library like `focus-trap-react`)
 - [ ] Dynamic content updates use `aria-live`
 - [ ] `prefers-reduced-motion` is respected for animations
 ## Related Skills
 - `frontend-patterns` — general React component and state patterns
 - `design-system` — design token and component consistency
 - `motion-ui` — animation patterns with accessibility considerations